Cathode sputtering processes are those in which a target, formed of a sputtering material which is to be deposited on the surface of a substrate, is supported in a vacuum chamber in a position facing the surface of the substrate to be sputter coated with the target material. A negative potential is then applied to the target to produce an electric field which will cause electrons to be emitted from the target surface toward a remote anode. In a magnetron sputtering apparatus, a magnetic field is imposed with magnetic lines of force which intersect the electric field, diverting the electrons along spiral paths whereby they become trapped over the target surface. The electrons ionize a sparse inert gas within the chamber causing positive gas ions to be formed, particularly where the electrons are densely trapped, forming a glow or plasma of high ion concentration. The ions are attracted to the negative target surface which they bombard, ejecting small particles of the sputtering target material. The particles of sputtering material emitted from the target surface strike and adhere to the surface of the substrate forming the sputtered coating.
Sputtering targets are formed of specialty materials which usually have stringent requirements for purity, crystal size, structure, etc. They are usually cast or otherwise molded and then machined to provide a sputtering surface of a specific macroscopic shape which will assume a particular geometric relationship with a substrate to be coated when mounted in a sputtering apparatus. Its surface is typically well polished. Invariably the sputtering surfaces of newly installed targets will contain oxides and other contaminants.
Before the sputter coating process can efficiently and effectively take place, however, the surface of a target, which has been newly installed in a sputtering apparatus, must be preconditioned. Preconditioning requires a cleaning of the target surface and a microscopic roughening of the target surface to produce a texture which will modify the electric field and draw the plasma close to the target surface. Typically, this preconditioning of the target surface, called "burn-in", is performed by sputtering in the sputtering chamber. During burn-in, the target surface is bombarded with ions which clean the surface, and then further bombarded until target material, which is usually more susceptible to removal by sputtering from crystal grain boundaries, is preferentially sputtered therefrom, forming a roughened texture or microstructure. This microstructure modifies the way the electric field lines form at the sputtering surface and tends to cause the plasma to draw closer to the surface, increasing the sputtering rate which a given electrical potential will produce.
A sputtering target, over its effective-coating lifetime, is usually operated either at constant power, or in a manner which maintains a constant deposition rate onto the surface of the substrate to be coated. The delivery of the desired sputtering power calls for operation of the power supply at some operating voltage, typically 500 to 600 volts, which will often decline over the useful sputtering life of the target. Such operation will require, however, an often undesirably high voltage to be applied to new target in order to condition its surface for sputtering onto substrates. Such a high voltage is required to develop the desired power level needed to effectively sputter clean a new, smooth, contaminated target. The initial and highest voltage to be applied to this target is a voltage which is required to efficiently remove surface oxides and other surface contaminants from the target surface during this initial preclean period. This voltage may be in the range of 900 volts where a typical operating voltage may be, for example, 600 volts.
Following the sputter precleaning process, additional sputtering of the target surface transforms the normally smoothly machined target surface to a roughened surface by the preferential removal of material from the grain boundary areas of the surface of the metallic target to expose the undisturbed crystal structure of the target. This is sometimes referred to as the target "burn-in." Until the surface of the target is so roughened, the plasma, or cloud of ions formed over the target surface, is insufficiently close to the target surface for electron and ion current to flow sufficiently. As a result, the "burn-in" voltage is also high, at least at the beginning of the burn-in process.
After a microtexture is developed by preconditioning the sputtering target surface, normal sputter coating of the target can take place in which the sputtering of material occurs at an effective rate and at a sufficiently safe voltage to deposit the coating onto the surface of the substrate. During the normal sputtering process, as material is sputtered from the target surface, an erosion groove will generally form in the surface of the target as target material is consumed and the target is ultimately expended.
During the various phases, which have heretofore been typically performed by sputtering in a sputter coating chamber, the voltage applied to the target begins at a high level and then gradually declines. This wide range of voltage variation over the life of the target is undesirable. A wide variation in the voltage produces more stringent demands upon the operation of the sputtering power supply, while a small variation allows the supply to operate over the life of the target closer to the design power limits of the power supply. Where the initial voltage must be considerably higher than the normal operating voltage needed for the sputtering process, design of the power supply to accommodate high initial voltages increases the cost of the power supply. Higher voltages also increase the risk of voltage breakdown which can damage sensitive devices on the substrate. Furthermore, the wide variation in voltage can result in inconsistencies in the deposition parameters throughout the coating processes, adversely affecting the film characteristics on the substrate over the target life.
Furthermore, the initial preparation of the target surface by sputtering, to texture the target through a target "burn-in" phase to develop surface irregularities having sufficient minute features the trap the plasma, typically takes several hours of operation. This several hour burn-in period pre-empts the use of the valuable sputtering apparatus for the productive coating of wafers, and in addition, involves several thousand dollars in operating costs.
Accordingly, there is a need to provide the surface of sputtering targets with the desired texture necessary for efficient sputter coating therefrom without subjecting the target to the high preconditioning voltage and long burn-in period heretofore required.